CHERBOURG, France  A heavily guarded ship carrying a large shipment of U.S. weapons-grade plutonium docked in the French port of Cherbourg on Wednesday, defying protesters who say the shipment is vulnerable to terrorist attack.

The Pacific Pintail sailed slowly into the northwestern port at around dawn, escorted by helicopters. Armed police kept guard as the plutonium was unloaded before being taken by road to a nuclear reprocessing plant on the nearby La Hague Peninsula.

The environmental group Greenpeace, which staged protests in the last few days, watched from small boats as the boat docked but did not interfere. A French court ruling on Tuesday barred protesters from going within 100 meters (yards) of the shipment.

The delivery is part of a postCold War agreement between the United States and Russia to get rid of plutonium from excess nuclear warheads.

Greenpeace says the shipment was 140 kg (308 pounds) of plutonium. A spokesman for the U.S. Security Administration said the amount being transported was 125 kg.

After it is unloaded at the La Hague plant, the plutonium will be driven nearly 1,000 km (660 miles) to a factory in southeastern France for recycling.

Although the shipment will have an armed guard and appears not to have alarmed the French public, Greenpeace fears it could be attacked or intercepted by nuclear terrorists.

"This shipment of weapons-grade plutonium is a wake-up call to the world," said Tom Clements of Greenpeace International. "The military nature of the arrival in France clearly demonstrates that nuclear weapons materials are a threat to global security and have no place in commerce."

French state-owned nuclear energy firm Areva, which is being paid to reprocess the plutonium, says it is safe.

"The plutonium ... is shipped in casks that comply with the regulation of the International Atomic Energy Agency (IAEA). Its transport is the object of the strongest safety and security measures," Areva said in a statement.

An Areva unit called Cogema will recycle the plutonium into nuclear fuel at its Cadarache and Marcoule plants in southeastern France. This will then be shipped back to the United States for use in an electricity-generating reactor.

The agreement is part of the U.S. Department of Energy's program to turn plutonium from the "excess" nuclear warheads into mixed-oxide (MOX) plutonium-uranium enriched fuel.

A spokesman for the U.S. National Nuclear Security Administration, an independent arm of the U.S. Energy Department, accused Greenpeace of scaremongering.

"This agreement is for peaceful cooperation. Anytime when weapons-grade plutonium can be destroyed, it is good," the spokesman said. "We have been planning this for two years and we're confident enough has been done to protect it."

Greenpeace protesters bolted a heavy truck to the road leading to La Hague on Tuesday and chained themselves to the vehicle to try to stop the delivery.

Police used chain cutters to cut free the protesters and later removed the truck. Under Tuesday's court ruling, any protester who goes within 100 meters of the shipment faces a 75,000 euro (US$92,280) fine.